Investments - Schedule of Available-for-Sale Investments (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018
Investment [Line Items]
Available-for-sale investments	$ 234.9	$ 419.0
Certificates of deposit and time deposits
Investment [Line Items]
Available-for-sale investments	3.7	169.6
Corporate debt securities
Investment [Line Items]
Available-for-sale investments	151.1	158.4
Government securities
Investment [Line Items]
Available-for-sale investments	59.0	65.8
Asset-backed securities
Investment [Line Items]
Available-for-sale investments	$ 21.1	$ 25.2

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant